Means for focusing beams of charged particles



$.RUH HUUM J. A. HANSEN MEANS FOR FOCUSING BEAMS OF CHARGED PARTICLES Sept. 8, 1964 Filed April 30. 1962 'INVENTOR* JOHN A. HANSEN AGENT United States Patent 3,148,265 MEANS FOR FOCUSING BEAMS 0F CHARGED PARTICLES John A. Hansen, Granby, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Apr. 30, 1962, Ser. No. 191,012 6 Claims. (Cl. 219117) My invention relates to working materialswith an intense beam of charged particles. More particularly, my invention relates to a method and apparatus for controlling the focusing of an electron beam in an apparatus which uses such beam to machine or perform other operations on any material.

Electron beam machines, as they are generally known, are devices which use the kinetic energy of an electron beam to work a material. US. Patent No. 2,793,281, issued May 21, 1957, to K. H. Steigerwald, discloses such a machine. These machines operate by generating a highly focused beam of electrons. The electron beam is a welding, cutting and machining tool which has practically no mass but has high kinetic energy because of the extremely high velocity imparted to the electrons. Transfer of this kinetic energy to the lattice electrons of the work piece generates higher lattice vibrations which cause an increase in the temperature within the impingement area sufficient to accomplish work.

It is a fact that a beam of highest power density power per unit of impingement area-is more effective. That is, a high power density beam can accomplish the required work in the shortest possible time and thus minimize heat conductivity so that the material adjacent to the area being worked is relatively unaifected. In order to obtain high power density, precise electron optics have to be applied in focusing the beam. Beam power density is defined as:

where From the above discussion and Equation 1 it becomes apparent that optimum conditions for working material with an electron beam require the smallest possible spot size consistent with the type of electron optics used. It should be noted that the electron optical system employed is modified in accordance with the type of operation to be performed. That is, the minimum spot size obtainable in, for example, an electron beam Welder would be too large for a cutting operation.

The basic formula for the electron beam current in an electron optical system has been derived by Langmuir and is set forth on page 750 of Electron Optics and the Electron Microscope, published by John Wiley and Sons Incorporated, New York, in 1957, and reads:

where:

C =depends only on the electron optics and filament heating d diarneter of beam impingement on work piece.

Solving this equation for d and combining with the beam impingement area is found to be:

7r i 3/4 'o This equation shows that the spot size and thus the beam power density is dependent on acceleration voltage, beam current and the electron optics of the machine.

To obtain maximum focusing, i.e., smallest obtainable spot size, prior art methods used a trial and error approach. That is, prior to my invention, a target such as a tungsten disc was placed under the electron beam at the same height at which Work was to be performed. The beam was then allowed to impinge on this target while the spot size was observed visually through an optical system comprising a microscope. This method has several inherent disadvantages. First, as the beam approaches focus, the beam power density increases and the beam begins to destroy the surface of the stationary target. This tends to prevent accurate focusing. Secondly, when an irregularly shaped piece was to be worked, this time consuming operation has to be repeated at each different height of the work piece. Also, because of the intense light given off from the target, this method of focusing is very irritating to the eye.

My invention overcomes the above disadvantages by providing a novel method and apparatus for focusing an intense beam of charged particles.

It is therefore an object of my invention to focus a beam of charged particles.

It is another object of my invention to focus a beam of charged particles rapidly and accurately.

It is yet another object of my invention to automatically focus a beam of charged particles.

It is another object of my invention to focus a beam of charged particles without damaging the surface of the element used to sense the focusing.

It is also an object of my invention to focus a beam of charged particles by sensing the intensity of the beam at various points along the beam axis to determine the focus point thereof and to thereafter compare an indication of the actual focus point with an indication of the gesired focus point to provide for adjustment of beam ocus.

It is still another object of my invention to determine the focus point of a beam of charged particles by sensing the relative location of a hot spot generated by impingement of the focused beam on a rotating target.

It is another object of my invention to obtain an accurate and repeatable degree of defocus of a beam of charged particles.

These and other objects of my invention are accomplished by positioning a sensing element comprising a wobble mounted disc at the spot where work is to be performed by a focused beam of charged particles, and rotating the disc around its skewed axis at a high rate of speed and in synchronism with a reference alternating voltage. The center line of rotation of the disc is parallel to the beam and at some distance from the beam so that the charged particles will impinge on the disc along a circular path close to the edge thereof. The disc is positioned so that the desired focus point will be the average level of the disc. As can be seen from Equations 1 and 2 above, large spot size means low beam power density. Therefore, a high power density beam will impinge upon the disc only at the point where the beam is focused and will thus produce a hot spot on the synchronously rotating wobble disc only at levels where the beam is focused. For example, when the beam is focused at the desired level, two hot spots will be generated which are apart on the surface of the disc. The relative locations of the hot spots are sensed by a photocell and compared with an alternating reference voltage. The phase relationship between the alternating reference voltage and the output signals from the photocell provides an indication of the actual focus point of the beam. This information can then be used to cause automatic adjustment of beam focus to the desired point.

My invention may be better understood and its numerous advantages will become apparent to those skilled in the art by reference to the accompanying drawing in which like reference numerals apply to like elements in the various figures and in which:

FIGURE 1 is a schematic view of an apparatus capable of being employed in the practice of my invention.

FIGURE 2 is an enlarged view of a sensing element which can be used with the apparatus of FIGURE 1.

FIGURES 3 through 5 show voltage wave-forms which appear at various points in the circuit shown in FIGURE 1 when the sensing element is in the position shown in FIGURE 2.

FIGURE 6 is a view of the sensing element shown in FIGURE 2 in a second position.

FIGURES 7 and 8 show the phase relationship of the voltages in the circuit when the sensing element is in the position shown in FIGURE 6.

FIGURE 9 is a view of the sensing element shown in FIGURE 2 in a third position.

FIGURES 10 and 11 show the phase relationship of the voltages in the circuit when the sensing element is in the position shown in FIGURE 9.

FIGURE 12 is a view of an alternative sensing element that may be used in the practice of my invention.

Referring now to FIGURE 1, an electron beam machine is indicated generally as 10. The machine comprises a vacuum chamber 12 containing a work piece 14 positioned on a movable table 16. The machine also comprises an electron beam column 18 containing a source of electrons, beam forming means and beam focusing means. The source of electrons comprises a directly heated cathode or filament 20 which has a negative acceleration voltage supplied thereto. An apertured anode 22 is positioned in the electron beam column 18 between the cathode and the work piece. The anode is connected to the case of the machine which is grounded at 24. The difference in potential between the cathode 20 and anode 22 causes the electrons emitted from the cathode to be accelerated down column 18. The electrons are focused into a beam indicated generally at 26, by an electron optical system comprising adjustment coils 28 and 30 and diaphragm 32. The beam is then focused at the desired level, in the most usual case, by a magnetic lens 34.

Under operating conditions, the beam impinges on work piece 14 where it gives up kinetic energy in the form of heat. The work piece 14 may be moved beneath the beam by movable table 16 and the beam may be deflected over the work piece by means of deflections coils 36. Positioned adjacent cathode 20 is a control electrode 38. This control electrode is normally maintained at a voltage which is more negative than the voltage applied to the cathode. The magnitude of this bias or voltage difference is variable by adjusting a bias voltage control, not shown. The control electrode while aiding in the focusing of the beam also performs the same function as the grid in an ordinary triode vacuum tube.

When it is desired to focus the electron beam at a particular level in chamber 12, a sensing device indicated gen erally at 40 is positioned so as to be impinged upon by the beam. At this instant, table 16 which carries work piece 14 has been moved to one side. The sensing device 40 is shown in FIGURE 2 and consists of a circular wobble mounted disc 42, which is driven by a synchronous motor 44, and a photocell 46. The device 40 is positioned so that the beam will impinge on disc 42 adjacent the edge thereof. The photocell 46 is shown located above the diametrically opposite edge of the disc from the beam impingement point. While this is the most convenient position for the photocell, other locations are, of course, possible.

Wobble disc 42 is a rapidly spinning member which is large enough to dissipate the absorbed heat without itself reaching excessive temperatures. The disc is driven by a synchronous motor so that the angular position thereof is always in a fixed phase relative to the alternating reference voltage from supply 50. If the current to magnetic lens 34 of the electron beam machine is initially within the desired range there will be generated on the wobble disc a circular pattern of hot and cool zones. Taking the example of a two pole synchronous motor, which turns one revolution per cycle of the supply voltage, the shaft position and hence the disc is always in a fixed phase relationship to the applied A.C. voltage from source 50. It is, therefore, a simple matter to attach the wobble disc to the shaft so that spaced hot spots will occur on the disc at the zero and 180 points of the sinusoidal AC. voltage applied to motor 44 from the source 50. This condition is illustrated in FIGURE 2 and represents the normal condition of correct value of the lens current to magnetic lens 34. The position of the hot zones will be sensed by the photocell 46. The output from photocell 46 which, for the focused condition of FIGURE 2, is shown in FIG- URE 4, is applied to a preamplifier 52 and thence to a limiter 54-. The limited photocell output signals, shown in FIGURE 5, are applied to a phase comparator 56 where they are compared to the AC. voltage from source 50. The limited signals from the photocell act as gating signals in phase comparator 56 so that the filtered output thereof is essentially the integral of the shaded areas of the AC. voltage supply as shown in FIGURE 3. The integrating or smoothing of the comparator output is accomplished by a filter 58 which may be any well known integrator circuit. Therefore, under the balanced condition of 180 spaced hot spots, as shown in FIGURE 2, the input to lens current control 60 is zero. Consequently, since no error signal will be supplied to lens current control 60, no adjustment of the beam focusing through variation of lens current, will occur.

Referring now to FIGURE 6, there is illustrated a condition wherein the beam is focused at a point above the desired working position. Under this condition, the hot spots on the wobble disc will both fall within the same semi-circular segment of the disc. Under extreme conditions, where the beam is focused at either the highest or lowest points along the beam axis scanned by the disc, the two hot spots will coincide and produce only one photocell output signal coinciding in phase with either the positive or negative peak values of the sinusoidal voltage from source 50. FIGURES 7 and 8 show the phase relation between the limited output of the photocell and the supply voltage from source 50. Since, as described above, the output of limiter 54 gates phase comparator 56, the input to the lens current control 60 for the condition illustrated in FIGURE 6 will be the integral of the shaded areas of the wave form of FIGURE 7. This is, of course, a negative error signal which indicates that the current supplied to magnetic lens 34 is too high. This negative error signal is applied to the lens current control 60 to cause a decrease in the lens current. Lens current control 60 may be a DC. amplifier and/ or a servo system coupled with a variable current source such as a saturable reactor.

Referring now to FIGURE 9, there is illustrated the converse situation to that illustrated in FIGURE 6. In FIGURE 9, the lens current is too low and therefore the beam is focused at a point below the desired Working position. Under this condition, the hot spots generated on the wobble disc will again both fall within the same semi-circular segment of the disc. However, this seg ment of the disc will be opposite to that upon which the hot spots will be formed if the beam is focused above the desired position. From an examination of FIG- URES 10 and 11, which represent the supply voltage and limiter output wave forms respectively, it can be seen that the input to lens current control 60, for the condition illustrated in FIGURE 9, will be a positive error signal. This positive error signal will, when applied to lens current control 60, cause an increase in the lens current.

It should be evident from the above explanation that the wobble disc is suitable only for focusing the beam at the average level of the disc. If focusing is desired at another level, it would be necessary to reposition the motor and wobble disc vertically. For focusing at various levels without repositioning of the motor it is possible to use a one revolution helix of a screw surface in place of the wobble disc. Such a one-turn screw surface is illustrated in FIGURE 12. Use of the one-turn screw surface, of course, would produce a single hot spot. However, the same circuits can be used as for the wobble disc except that a phase shifter would be inserted between the supply voltage source and the phase comparator. By shifting the phase of the supply or reference signal it is possible to automatically focus the beam at any level between the lowest and highest points of the one-turn screw surface even though only one photoelectric signal will be generated per cycle. That is, by shifting the phase of the reference voltage, the focus point or point where there is coincidence between the hot spot and zero level of the reference voltage can be raised or lowered. As an alternative embodiment to the circular disc shown in FIGURE 2, since it may be desirable to reduce the thermal conductivity of the disc in the angular direction, a toothed disc or spoked member may be used in place of the solid disc.

While the preferred embodiment of my invention has been shown and disclosed, various modifications and substitutions may be made without deviating from the scope and spirit thereof. For example various other circuits for automatically focusing the beam might be utilized. Also, while I have disclosed my invention in terms of varying the focusing by adjusting the lens current of a magnetic lens, various other parameters may be adjusted either singly or along with lens current. Referring back to Equations 1 and 2, it is obvious that either beam current or acceleration voltage might be varied without departing from the teaching of my invention. In this respect beam current might be varied by adjusting the control electrode voltage or filament temperature. Also, if an electrostatic lens is used to focus the beam, the focusing voltage may be controlled automatically using the principle of my invention. Under certain circumstances, it may be desired to Work with a defocused beam. For example, it may be necessary to weld with a machine having an electron optical system suitable only for cutting. Under these circumstances, my invention may be used to achieve focusing at a point other than the position where the work is to be performed. The beam impinging upon the work would, therefore, be defocused to a controllable degree. Thus, my invention has been described by way of illustration rather than limitation and accordingly it is understood that my invention is to be limited only by the appended claims taken in view of the prior art.

I claim: 1. An electron beam welding and cutting machine comprising:

means for producing a beam of electrons, means for varying the focus of said beam, means for positioning a work piece in line with said beam, a beam scanning member positionable in line with said beam, means for causing said scanning member to scan a region along the beam axis which includes the level at which the work piece is to be positioned, means associated with said scanning member for sensing where the focused beam impinges thereon, means responsive to said sensing means for comparing the actual beam focus position with an indication of the desired beam focus position, and means responsive to said comparing means for varying the output of said means for varying beam focus. 2. The apparatus of claim 1 wherein the beam scanning member is a wobble mounted disc.

3. The apparatus of claim 1 wherein the beam scanning member is a one-turn screw surface.

4. Apparatus for sensing the focusing of an intense beam of charged particles comprising:

means for scanning a region along the axis of an intense beam of charged particles; means for sensing hot spots produced on said scanning means by impingement of the beam thereon at the focal point thereof; means connected to said sensing means for generating an electrical signal whenever a hot spot on said scanning means is sensed by said sensing means, said generated signal being indicative of the point of impingement of the focused beam on said scanning means; means for generating a reference signal synchronized with the scanning of said scanning means; and comparing means responsive to said reference signal and to the signal indicative of the point of impingement of the focused beam on the scanning means for producing an indication of beam focus position. 5. The apparatus of claim 4 wherein the scanning means comprises a wobble mounted disc.

6. The apparatus of claim 4 wherein the scanning means comprises a one-turn screw surface.

References Cited in the file of this patent FOREIGN PATENTS 1,072,763 Germany Jan. 7, 1960 

1. AN ELECTRON BEAM WELDING AND CUTTING MACHINE COMPRISING: MEANS FOR PRODUCING A BEAM OF ELECTRONS, MEANS FOR VARYING THE FOCUS OF SAID BEAM, MEANS FOR POSITINING A WORK PIECE IN LINE WITH SAID BEAM, A BEAM SCANNING MEMBER POSITIONABLE IN LINE WITH SAID BEAM, MEANS FOR CAUSING SAID SCANNING MEMBERTO SCAN A REGION ALONG THE BEAM AXIS WHICH INCLUDES THE LEVEL AT WHICH THE WORK PIECE IS TO BE POSITIONED, MEANS ASSOCIATED WITH SAID SCANNING MEMBER FOR SENSING WHERE THE FOCUSED BEAM IMPINGES THEREON, MEANS RESPONSIVE TO SAID SENSING MEANS FOR COMPARING THE ACTUAL BEAM FOCUS POSITION WITH AN INDICATION OF THE DESIRED BEAM FOCUS POSITION, AND MEANS RESPONSIVE TO SAID COMPARING MEANS FOR VARYING THE OUTPUT OF SAID MEANS FOR VARYING BEAM FOCUS. 